improve log output of fromager for debugging resolver failures

[dhellmann]

We get errors like

Traceback (most recent call last):^M
  File "/opt/app-root/lib64/python3.11/site-packages/fromager/bootstrapper.py", line 134, in bootstrap^M
    self.bootstrap(dep, RequirementType.INSTALL)^M
  File "/opt/app-root/lib64/python3.11/site-packages/fromager/bootstrapper.py", line 108, in bootstrap^M
    ) = self._prepare_source(req, source_filename, resolved_version)^M
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^M
  File "/opt/app-root/lib64/python3.11/site-packages/fromager/bootstrapper.py", line 227, in _prepare_source^M
    self._handle_build_requirements(^M
  File "/opt/app-root/lib64/python3.11/site-packages/fromager/bootstrapper.py", line 265, in _handle_build_requirements^M
    raise ValueError(^M
ValueError: could not handle build-system dependency torch~=2.3.1 for [(<RequirementType.TOP_LEVEL: 'toplevel'>, <Requirement('docling[tesserocr]')>, <Versio\
n('2.3.1')>), (<RequirementType.INSTALL: 'install'>, <Requirement('docling-ibm-models<3.0.0,>=2.0.3')>, <Version('2.0.3')>), (<RequirementType.INSTALL: 'inst\
all'>, <Requirement('torchvision<1,>=0')>, <Version('0.18.1')>)]^M

which are hard to read. We can improve the formatting.

We should also ensure that we log that information as we start each step of bootstrapping, since the recursion means that for some packages the beginning of processing may be very far away in the log from the point where their build step fails.

[dhellmann]

The basic ideas for this are done. We can track other improvements separately.